A Long Term Evolution-Advanced (LTE-A) system supports transmission bandwidth greater than 20 MHz, and even 100 MHz. To support wider bandwidth and be more compatible with the LTE system, two or more carriers may be aggregated. A terminal may receive or transmit data on one or more carriers simultaneously according to capabilities of the terminal and service requirements. An LTE-A terminal capable of receiving or transmitting data through bandwidth greater than 20 MHz can receive or transmit data on multiple carriers simultaneously; but an LTE terminal can receive or transmit data on a single downlink carrier compliant with the Rel-8 specifications and the corresponding uplink carrier only.
Carrier aggregation refers to aggregation of multiple carriers of a continuous or discrete spectrum. Each carrier may be an LTE carrier, namely, a backward-compatible carrier, which is accessible by an LTE terminal. Or, some carriers are non-LTE carriers, namely, non-backward-compatible carriers, which are not accessible by an LTE terminal. Among the carriers that are aggregated, the bandwidth of one carrier may be the same as or different from the bandwidth of another. For example, a 5M carrier is aggregated with a 10M carrier. In practice, carrier aggregation depends on capabilities of a terminal, service requirements and network configuration. For example, for N downlink carriers and M uplink carriers configured in the network, a high-capability terminal may use N downlink carriers and M uplink carriers, and a low-capability terminal may use N downlink carriers or part of the downlink carriers, but can select only one or some of the M uplink carriers for transmitting data.
For an LTE Frequency Division Duplex (FDD) system, a downlink carrier band always uniquely corresponds to an uplink carrier band, thus forming a pair of uplink and downlink carriers of the FDD system. The mapping relation between the uplink carrier and the downlink carrier is configured by the system. The transmission function of certain channels on the uplink carrier depends on the signaling transmitted by the downlink carrier. For example, a Dedicated Broadcast Channel (D-BCH) of the LTE downlink carrier bears information of the uplink Random Access Channel (RACH) and time-frequency resource information of the uplink Physical Uplink Control Channel (PUCCH).
In a multi-carrier system, a terminal-specific primary carrier concept and some applications are put forward. A primary carrier is a carrier where a terminal needs to receive and detect data frequently and which transmits information controlling whether data needs to be received by other downlink carriers or not. The primary carrier facilitates other mechanisms such as the application of the method of scheduling Discontinuous Reception (DRX)/Discontinuous Transmission (DTX) and Voice over IP (VoIP) services continuously. In addition, other carrier-related control information may also be carried on the primary carrier, for example, information about the frequency location of other carriers, and bandwidth. For a terminal capable of receiving more than one carrier simultaneously, continuous detection of great bandwidth (namely, multiple carriers) consumes too much power. In the concept of the terminal-specific primary carrier, the terminal needs to detect only the primary carrier. In this way, the battery consumption of the LTE-A terminal is reduced, and the requirement for using multiple carriers to transmit data is still fulfilled.
However, in the existing multi-carrier FDD system, the selection of the primary carrier of the terminal is independent of the uplink Transmitting (TX) carrier. That is, the selection of the primary carrier does not depend on the uplink carrier on which the data is transmitted. In an LTE system, the broadcast signaling in the corresponding downlink carrier carries the random access information of the uplink carrier and the information about the time-frequency resources occupied by the control signaling of the uplink carrier, and such information changes frequently. For an LTE-A system that supports multiple carriers, if the downlink primary carrier selected by the terminal does not correspond to any carrier of the data sent on the uplink, the terminal needs to detect the broadcast information of the downlink carrier corresponding to the uplink carrier frequently to obtain the random access information related to the uplink carrier transmission or information about the time-frequency resources occupied by the control signaling; further, the terminal needs to switch and detect the downlink carrier corresponding to the uplink TX carrier constantly, which increases the battery consumption of the terminal.